1. Field of the Invention
The present invention relates to a diaphragm control apparatus of an interchangeable lens camera, and in particular, relates to a diaphragm control apparatus which enables an adjustable diaphragm of an interchangeable lens (attached to a camera body) to be controlled by the camera body during exposure in an interchangeable lens SLR camera system.
2. Description of the Related Art
Diaphragm mechanisms (i.e., aperture mechanisms) of conventional interchangeable lens SLR camera systems are configured in a manner so that a diaphragm control bar, which constitutes an element of a diaphragm control mechanism of a camera body, moves a diaphragm operatively-associated rod of the interchangeable lens that is provided to drive a diaphragm mechanism of the interchangeable lens. In the case where the diaphragm control mechanism is powered by a motor of a mirror drive mechanism or a shutter charge mechanism, the diaphragm can be controlled only in a single direction due to the structure of the diaphragm control mechanism. For instance, the diaphragm control mechanism controls the operation of the diaphragm in such a manner as to drive the diaphragm in a diaphragm stop-down direction from an open-aperture (full-aperture) state and subsequently stops the stop-down movement of the diaphragm with a ratchet when the diaphragm is stopped down to a previously-set aperture value (i.e., f-number), and accordingly, the f-number of the diaphragm cannot be adjusted afterwards.
In such conventional diaphragm mechanisms, when a live-view operation, in which image data obtained from an image sensor (image pickup device) is displayed on a display monitor  in real time, or a movie shooting operation is performed, the f-number cannot be adjusted from an initially-set f-number. In order to enable an f-number adjustment during a live-view operation, the assignee of the prevent invention has proposed an invention for controlling the operation of an adjustable diaphragm so as to open and shut the adjustable diaphragm with the use of a diaphragm drive motor serving as a driving source of a diaphragm control mechanism (Japanese Unexamined Patent Publication 2008-197552). This related invention makes it possible to make an adjustment to a diaphragm setting during a live-view operation or a movie shooting operation.
In conventional interchangeable lenses, the open-aperture reference position of the diaphragm operatively-associated rod varies depending on the f-number at open aperture. Therefore, when an interchangeable lens is attached to a camera body, the amount of movement of the diaphragm control rod, which is provided in the camera body, by the diaphragm operatively-associated rod varies depending on the type of interchangeable lens attached to the camera body. In the case where a stepping motor is used as a driving source of the diaphragm control mechanism, the stepping motor is forced to rotate in association with movements of the diaphragm control rod; however, the amount of rotation of the stepping motor varies depending on the type of interchangeable lens attached to the camera body. As a result, the stepping motor (the rotor thereof) rotates from the initial detent position thereof, making the stop position of the stepping motor uncertain. Additionally, in conventional interchangeable lenses, it is sometimes the case that the open-aperture reference position of the diaphragm operatively-associated rod, i.e., the initial position thereof relative to a camera body when an interchangeable lens is attached to the camera body, may be erroneous due to mechanical  error or assembling error, etc. In such a case also, due to this positional error, it is sometimes the case that the stop position of the stepping motor deviates from the preset initial position thereof.
Stepping motors that can be utilized as diaphragm drive motors are usually of a type which is driven to rotate by steps in one direction, normally by being repeatedly energized with a plurality of excitation patterns in order. In this type of stepping motor, if the stop position and the phase of the excitation pattern do not coincide with each other, a problem occurs with the stepping motor possibly rotating in a direction reverse to the required rotational direction, or even not rotating at all, which causes a mismatch between the number of excitations and the number of steps for driving the stepping motor, thus causing an error in f-number control.
In the case of bringing the stepping motor into a free state by cutting off the power applied to the stepping motor after driving the stepping motor stepwise until the motion of the diaphragm control mechanism is mechanically restricted to detect the initial excitation pattern of the stop position (herein referred as the “origin position”) of the stepping motor, there is a possibility of the stop position of the stepping motor becoming unstable upon the stepping motor entering a free state because, e.g., some components of the diaphragm control mechanism may be deformed by the torque and the inertial force of the stepping motor, and the restoring force of such elements may force the stepping motor to rotate in reverse upon the stepping motor entering a free state.